1. Field of the Invention
The present invention relates to an integrated circuit and a method of fabricating the same, and more particularly to a metal oxide semiconductor (MOS) device and a method of fabricating the same.
2. Description of the Prior Art
The process of forming a complementary metal oxide semiconductor (CMOS) image sensor is compatible to the process of forming a CMOS transistor. Consequently, the CMOS image sensor can be fabricated with other peripheral circuits on a same chip. Thus, the power consumption and the costs of fabricating the image sensors are significantly reduced. In the recent years, the CMOS image sensor has replaced a charge-coupled device (CCD) in the low-end application and become more and more predominant.
The CMOS image sensor includes a photodiode and a plurality of transistors. The photodiode is constructed by a P-N junction constituted of an N-doped region and a P-substrate. The transistors are N-type MOS transistors having N-type gates (N-poly NMOSs). Currently, the CMOS image sensor includes a 3-transistor (3-T) configuration and a 4-transistor (4-T) configuration.
A typical 3-T configuration refers to the CMOS image sensor including a reset transistor, a source follower transistor, a select transistor, and a photodiode. However, the 3-T configuration brings about high dark current, increases readout noises and adversely affects the image quality, thus reducing the performance of the devices. Hence, the 4-T configuration is implemented more frequently.
FIG. 1 schematically illustrates a conventional 4-T CMOS image sensor.
In FIG. 1, the CMOS image sensor includes a substrate 100, a transfer transistor 102 disposed on the substrate 100, a reset transistor 104, a source follower transistor 106, a select transistor 108, a photodiode 110 disposed in the substrate 100, a floating node 112 disposed in the substrate 100, and a P-well 114 disposed in the substrate 100. Since the 4-T CMOS image sensor includes the transfer transistor 102, the high dark current of the 3-T CMOS image sensor can be avoided.
In the 4-T CMOS image sensor, the spacer material deposited on the photodiode region is frequently kept, so as to protect the photodiode region from being damaged during the etching and the formation of the spacers and to avoid the dark current from increasing. Generally, the thickness of the spacers exceeds 1000 angstroms, and the spacers are often made of silicon nitride. Thereby, excessive silicon nitride is left in the photodiode region, which blocks lights emitting into the photodiode region and impairs the sensitivity of the sensor. On the contrary, if the spacers are made of silicon oxide, a subsequently formed anti-reflective layer is disposed far away from the photodiode region due to the excessive thickness of silicon oxide. Hence, the reflection cannot be effectively suppressed.